There are many existing and emerging applications of high power lasers in industrial, medical and scientific processes. Conventionally, solid state and gas lasers, with bulk-optical cavities, are used in these applications, and the outputs of these lasers are generally free space, collimated beams. In most cases, the laser source can be very large, and often constitutes the largest single element of an instrument or machine. In using laser systems in industrial applications, it is often difficult to position the large laser very close to the intended target of the laser beam and therefore bulk-optic beam steering is required. Such beam steering includes specially designed, low loss, high reflective and high-damage threshold mirrors, mounted and positioned with precision and on a heavy and vibration-isolated surface to enable a robust, reliable performance. In many applications such as semiconductor inspection, the inspection tool is just one of many instruments and machines used within the semiconductor manufacturing process, and space on the manufacturing floor—very often being in an expensive, clean-room environment—costs a premium. Therefore, reduction of the instrumentation footprint is highly beneficial. In other industrial applications, such as laser materials processing, the application environment can very often be noisy, dirty and a challenge to the operation of a laser source. In this application additional measures are often required to protect the laser and beam steering optics from the hostile working environment.
Fibre delivery of the laser beam is a clear and interesting option, enabling the laser source to be positioned remotely from the target space and enabling a compact optical beam delivery head (optical head) to be installed within the instrument, with the large laser source being positioned, along with any power supplies and cooling systems, outside an instrument and clean room environment. With conventional bulk lasers, fibre delivery involves launching of the laser output beam into an optical fibre. This is very difficult to achieve, especially in high-power laser applications. Most applications require a good beam quality of the laser, which requires a single mode fibre to deliver the beam. In reality, a single mode fibre has a core diameter of less than 15 μm, very often less than 10 μm, and efficient and stable launching of a beam into this aperture is difficult to achieve. Furthermore, in high-power applications, launching of such high intensities into a fibre will damage the fibre facet.
In addition, applications of UV fibre lasers cannot use a conventional optical fibre to deliver the beam since the UV is absorbed by the fibre. One option that has been considered is to use Hollow Core Photonic Crystal Fibres (HCPCF's) in which the light is mostly guided within an air-core of the fibre. The use of HCPCF's does not solve the problem of launching the light into the fibre and avoiding facet damage at high powers. However, HCPCF's have two benefits—they enable the propagation of UV radiation with relatively low loss and also reduced fibre nonlinearity by a factor of approximately 1000 in comparison to conventional glass-guided optical fibres.
Fibre lasers clearly have a significant advantage over conventional bulk lasers, since the optical beam is already within the fibre and no launching optics are required. Most high-power fibre lasers, particularly in the pulsed lasers, use a Master Oscillator Power Amplifier (MOPA) configuration, in which the output of a low-power fibre oscillator is amplified in a series of high-power fibre amplifiers. In principle, the output of a fibre laser or fibre amplifier can be delivered directly to the intended target through an output fibre. However, in short pulse applications the nonlinear effects of the fibre and amplifier prevent this. Short optical pulses are generally defined (and defined herein) as pulses having a duration of less than 10 ns (10−8 seconds). Reduction of fibre nonlinearity is a major challenge in any fibre-based system, particularly when short pulses are required at relatively high peak intensities.
In cw and long pulse applications, there is little issue with the use of additional lengths of fibre at the amplifier output, since nonlinear effects can often be neglected owing to relatively low peak powers. However, for short-pulse fibre delivery, the nonlinear effects within an optical fibre prevent the delivery of high power pulses due to degradation of the pulses' temporal and spectral characteristics due to high order nonlinear effects such as self-phase modulation, which causes spectral broadening, and Raman scattering, which causes both spectral and temporal broadening.
HCPCF's and conventional optical fibres with larger core sizes (referred to as large-mode-area (LMA) fibres) have been used to reduce fibre nonlinearity. HCPCF's reduce the nonlinearity by several orders of magnitude, where LMA fibres reduce the nonlinearity, scaling with the area of the core. However, the use of LMA fibres for beam delivery in high-power pulsed applications is not a solution since even the largest single-mode core fibre (of 15-20 μm), results in significant nonlinear effects when high-peak power pulses are delivered.
The importance of reducing nonlinear effects can be illustrated by considering UV to generation from a short-pulse source. The use of short pulses to generate visible and UV radiation is a common approach, since the high peak powers attainable from short pulses provide efficient frequency conversion in nonlinear materials such as lithium triborate (LBO) and β-barium borate (BBO). However, efficient conversion within conventional nonlinear media often requires that the spectral bandwidth of the pulse is as narrow as possible—preferably with the pulses transform limited. Any high order nonlinearity within a delivery fibre or a fibre amplifier will result in spectral broadening; a relatively low nonlinearity will double the spectral bandwidth of a pulse and hence significantly reduce the conversion efficiency of that pulse. It is therefore important in all stages of amplification of the pulse to avoid or reduce fibre nonlinear effects.
For amplifying short optical pulses, it is possible to use Chirped Pulse Amplification (CPA), a technique developed for bulk-laser systems, in which a short pulse from an oscillator is stretched in a fibre or bulk optic stretcher, amplified and then compressed. In this instance, the amplified, long pulses can also be delivered by an optical fibre to a remote probe where they are subsequently re-compressed, as described in U.S. Pat. No. 6,249,630.
One of the main problems with the CPA approach is that, in order to amplify pulses without significant nonlinear distortion, the pulse must be stretched to a duration of typically greater than 10 ns. Therefore, to compress the stretched amplified pulse, a fairly long and complex compressor is required. Furthermore, in stretching and compressing the pulse, bulk optic components are typically required, preventing an all-fibre approach. This is difficult to make robust and involves significant loss which necessitates another stage of amplification in the MOPA. Furthermore, after amplification and fibre delivery, the compression stage also involves significant optical loss and, due to the relatively long pulse duration, the compressor itself is very large, meaning that the optical head itself has a large footprint. In general, the approach of CPA is not ideal and is also difficult to maintain in a compact, robust and lightweight form especially in a harsh operating environment.